Geomechanical Factors Affecting Geological Storage of CO2 in Depleted Oil and Gas Reservoirs

Abstract A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Potential leakage mechanisms reviewed include fault reactivation, induced shear failure of the caprock, out-of-zone hydraulic fracturing, and poorly sealed casing cements in enlarged, unstable boreholes. Parameters controlling these mechanisms include the upper and lower bounds of pressure and temperature experienced by the reservoir, the orientation and mechanical properties of existing faults, rock mechanical properties, in situ stresses, and reservoir depth and shape. Approaches to mitigate the likelihood of geomechanics-related leakage include the identification of safe upper limits on injection pressures, preferred injection well locations, review of historical records for reservoir pressures, temperatures and stimulation treatments, drilling program design to mitigate rock yielding in new wells, and assessment of wellbore integrity indicators in existing wells. Introduction In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO2) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO2 sequestration in geological media in terms of volume, duration, and minimum or null environmental impact(1). The first commercial scheme for CO2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 106 tonnes of CO2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1,000 m below the sea bed(2). In light of the economic benefits of enhanced oil recovery (EOR) derived from CO2 injection in oil reservoirs(3), these types of reservoirs will be attractive CO2 injection targets and, most likely, CO2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO2 storage in aquifers. An advantage of CO2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada(4) is an example of a large-scale application of EOR operations using anthropogenic CO2, in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone. A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The initial integrity of this "bounding seal" system is governed by geological factors. A considerable amount of effort has been devoted to the development of procedures for assessing fault seal capacity in potential hydrocarbon reservoirs(5).

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